Wafer cleaning system

ABSTRACT

A wafer cleaning apparatus provides two opposed brushes for brushing a vertically disposed wafer in a tank which can contain a process liquid. A pressure controller adaptively controls the pressure exerted by the brushes on the wafer to compensate for brush wear. Rim driving wheels engage the wafer periphery with a porous jacket coupled to a fluid delivery system, thereby simultaneously rotating and cleaning the periphery of the wafer. The apparatus includes a fluid delivery system for separately and independently delivering a plurality of constituents of a cleaning solution to the brushes, thereby ensuring that a freshly mixed cleaning solution reaches the wafer. The tank can be filled with a process liquid through which megasonic waves provided by a transducer can propagate and impinge upon the wafer thereby enhancing the cleaning of the wafer or the brushes.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/113,703 filed on Jul. 10, 1998, now U.S. Pat. No. 6,070,284,the contents of which are hereby incorporated by reference and claimsbenefit of U.S. Provisional Application, 60/073,637 filed on Feb. 41998.

FIELD OF THE INVENTION

This invention relates to the field of processing a substrate, and inparticular, to methods and devices for cleaning particulates and othercontaminants from, for example, a semiconductor wafer.

BACKGROUND

The process of manufacturing integrated circuits typically consists ofmore than a hundred steps during which hundreds of integrated circuitsare formed on a single semiconductor wafer. Generally, this processinvolves the creation of multiple layers on the semiconductor wafer Thislayering process forms the electrically active regions on the wafer'ssurface.

After completion of each layer, it is common practice to thoroughlyclean, rinse, and dry the wafer. Because the formation of each layerdepends on the accuracy and precision with which the previous layer wasformed, it is critical that each layer be free of contaminants orsurface impurities. In recent years, the smaller size, and henceincreased density of microelectronic components, has increased thedemand for more effective control of contamination.

A current method for removing contaminants from the surface of asemiconductor wafer uses a sponge roller to mechanically scrub thewafer's surface in the presence of a cleaning solution. In somepractices of this method, cleaning solution is applied either bydirecting a high pressure spray against the wafer or by drippingcleaning solution on the sponge rollers. Typically, the wafer ispositioned horizontally between two rotating sponge rollers. During thisscrubbing cycle, the wafer is rotated by an edge handling apparatus toensure that the sponge rollers engage and scrub substantially the entiresurface of the wafer.

Upon completion of the scrubbing cycle, the wafer is typicallytransferred to a rinsing-drying stage separate from the scrubbing stage.Typically, this transfer is effected by moving conveyor belts on whichthe wafer rests during the scrubbing cycle. These belts, whichfrictionally engage the bottom surface of the horizontally disposedwafer, are prone to leave residue on the wafer.

In this rinsing-drying stage, the wafer typically rests horizontally ona spindle chuck which spins at high speeds. A stream of de-ionized watersprayed onto the surface of the spinning wafer forms a thin film on thewafer's surface. In some cases, an acoustic transducer mounted on aprobe hydroplaning on the surface of this film generates megasonic wavesthat propagate through the film . These waves assist in dislodging anyparticles remaining after the scrubbing cycle. As the wafer spins, thede-ionized water, and any contaminant particles suspended within it, aredriven off the periphery of the wafer by centrifugal force. Thiseventually dries the wafer surface.

Commonly used wafer diameters have thus far been small enough so thatthe internal forces generated during the spinning of the wafer are, inmost cases, insufficient to stress or break the wafer. However, therecently introduced 300 mm wafers are so large that internal forcesgenerated during the spinning of the wafer significantly increase theprobability of breakage. This is further exacerbated by the fact that,in 300 mm wafers, the de-ionized water travels further to reach theperiphery. Consequently, spin drying 300 mm wafers requires highrotational speed to generate enough centrifugal force to rapidly movethe de-ionized water from the center to the periphery of the wafer.

The foregoing wafer cleaning method has additional disadvantages whichare unrelated to the size of the wafer. For example, megasonic energy isincident on the wafer only during the rinsing step. The lack ofmegasonic energy during the scrubbing step forecloses any synergisticoperation between the operations of brush scrubbing and of megasonicbombardment. In addition, the sponge rollers, which are never exposed tothe cleaning effect of megasonic energy, are prone to become dirty andto wear prematurely. This significantly increases the operating costs ofthe wafer-scrubbing machine.

Because megasonic energy effective for cleaning generally requirestransmission through a liquid, devices for megasonic cleaning providefor submerging a wafer in a liquid while exposing it to megasonicenergy. It has long been considered, therefore, that an apparatus forsimultaneously scrubbing a substrate while it is exposed to megasonicenergy would require immersion of moving mechanical parts. Any strayparticles generated by these moving mechanical parts would contaminatethe surrounding liquid. As a result, prior art cleaning devicestypically separate the brushing step from the megasonic cleaning step,as in the manner set forth above. Consequently, these cleaning devicesforego any synergistic effect associated with simultaneous brushing andmegasonic cleaning.

In the foregoing prior method, a cleaning solution consisting of one ormore liquids is typically mixed together and applied to the surface ofthe wafer, either by spraying the mixture onto the surface of the waferor by passing it through a sponge roller mounted on a hollow brush coreand allowing it to perfuse through the sponge roller. However, certainmixtures lose their cleaning potency soon after they are mixed. It ispreferable that such cleaning mixtures be applied to the surface shortlyafter they are mixed.

Since a typical cleaning cycle can use several different cleaningsolutions at different stages of the cycle, it is advantageous toprovide a system for quickly and efficiently switching from a firstcleaning solution to a second cleaning solution. In the case in whichthe cleaning solution is passed into a hollow brush core for perfusionthrough the sponge, an appreciable volume of cleaning solution remainsin the brush core. Thus, in order to change cleaning solutions, asignificant residual volume of the first cleaning fluid must be removedfrom the brush core before the second cleaning fluid can be used. Incases in which there exists an undesirable reaction between the old andnew cleaning solutions, this already time-consuming step is furtherlengthened by the need to flush the first cleaning solution with anon-reactive fluid rather than with the second cleaning solution.

The known practice of exposing the wafer to the second cleaning solutionby transporting it to a different scrubbing stage as an alternative tochanging the cleaning fluid in a scrubbing stage is unsatisfactory. Thisknown practice requires additional clean room space to accommodate theadditional scrubbing stage. In addition, the wafer transport mechanismfor transporting the wafer from one scrubbing stage to another canintroduce contaminants.

In the known method for cleaning a wafer with a cylindricalbrush-sponge, it is preferable to rotate the wafer as it is beingscrubbed to insure that all points on the surface of the wafer areexposed to the sponge roller. Current methods of rotating the waferinclude the use of rim driving wheels that rotate the wafer by engagingits periphery. These wheels, which are typically made of a hard plasticsuch as polyurethane, can undermine the entire cleaning process byleaving contaminants on the peripheral region of the wafer.

The prior art includes numerous attempts to avoid the foregoingdisadvantages. For example, in some wafer cleaning machines, a nozzlesprays liquid on the periphery of the wafer as the wafer is rotated inan effort to remove particles left behind by the rim driving wheels.Other wafer cleaning machines provide rim driving wheels made ofrelatively soft plastic materials.

The pressure exerted on the wafer by the sponge roller is an importantfactor in effective cleaning of wafers. If this sponge pressure is toolow, scrubbing is largely ineffective. If the sponge pressure is toohigh, particulates can inflict damage on the wafer by gouging itssurface. The applied sponge pressure is determined, in part, by thethickness of the sponge and by the thickness of the wafer. If eachthickness is known, one can readily apply a selected pressure by settingan appropriate distance between the sponge and the wafer.

The difficulty encountered in the foregoing method of controlling brushpressure is that these thicknesses are dynamically changing quantities.For example, as the sponge roller wears, its thickness can decrease.This typically results in a reduction in the brush pressure and anaccompanying reduction in cleaning effectiveness. It is known in the artto automatically adjust the distance between the sponge roller and thewafer according to a predetermined schedule based on empiricalobservations of the sponge roller's wear as a function of time. However,such a method, relying as it does on a statistically derived quantity,can easily result in application of pressure that is not the optimalpressure.

When the sponge roller ultimately wears out, it becomes necessary toreplace it. In current wafer cleaning machines, replacement isaccomplished by removing the worn sponge roller from the brush core andinserting the brush core through a fresh sponge roller. Since the spongeroller is typically a cylindrical tube of soft sponge-like materialhaving an inner diameter slightly smaller than the outer diameter of thebrush core, it is difficult and time consuming to insert the brush corethrough the sponge roller. The difficulty of this task frequentlyresults in damage to or contamination of the sponge roller. This resultsin increased operational expenses, because damaged sponge rollers mustbe discarded, or reduced yield, because contaminated sponge rollers areunlikely to clean wafers effectively.

SUMMARY OF THE INVENTION

A wafer cleaning system according to the invention includes a frame onwhich are mounted two brush devices and a wafer rotating mechanism. Thefirst brush device has a rotational axis parallel to a first face of awafer to be cleaned and the second brush device has a rotational axisparallel to a second face of this same wafer. Both brush devices areconfigured to rotate about their respective axes. The wafer rotatingmechanism is configured to engage a vertically oriented wafer andthereby rotate it about a rotational axis orthogonal to the first faceof the wafer.

In one embodiment, the frame contains a tank into which the wafer can bepartially immersed in a process liquid. In this embodiment, as the waferis rotated, selected sections of the wafer are alternately above orbelow the surface of the process liquid. A megasonic transducer can beprovided to generate megasonic waves which propagate through the processliquid and impinge upon those sections of the wafer immersed in processfluid, thereby cooperating with the brush devices in removingcontaminants from the wafer.

The wafer cleaning system can further include a pressure controllerwhich adaptively maintains a selected pressure between the brush deviceand the wafer to be cleaned. Such a pressure controller can beimplemented by providing two arms, each of which has a pivoting endcoupled to a drive mechanism and a free end having a brush devicemounted thereon. In response to the force exerted by one or both brushdevices on the wafer, the drive mechanism exerts a force on the arms.

Each brush device can have an associated fluid transport system fordelivering a plurality of fluids separately and independently to acleaning element forming the brushing surface of the brush device. Thecleaning element, which can be a porous sponge material such as PVA, canbe mounted on a brush core having fluid passages or channels arrangedthereon for delivery of a fluid to the cleaning element. A manifoldhousing having a plurality of manifolds enclosed therein can beprovided. Each such manifold operates independently of the remainingmanifolds to deliver a fluid from a fluid reservoir to a channel.

The brush core can include a sleeve engaged either frictionally, or bymechanical attachment, or by adhesion, with the cleaning element and aninner core within the sleeve. Both the inner core and the sleeve havefluid passageways which are in fluid communication with each other. Thisfluid communication can be achieved by aligning the fluid passagewayswith the perforations on the sleeve. In a preferred embodiment, thefluid passageways on the sleeve are perforations and those on the coreare channels or grooves on the surface of the core.

The inner core and the sleeve having a cleaning element mounted thereonare removably and replaceably engaged with each other. As a result, whenthe cleaning element requires replacement, the sleeve, with the cleaningelement engaged thereon, can readily be removed from the inner core andreplaced with a sleeve having a fresh cleaning element already mountedthereon.

A wheel member arranged for selective engagement of the periphery of theworkpiece can provide the workpiece rotating mechanism of the invention.Such a wheel member can include a wheel having a rim disposed forcontacting the peripheral region of the wafer and a porous or fluidpermeable jacket, which can be a porous or open cell sponge material,having an inner surface engaged with the rim and an outer surfaceexposed for engagement with the peripheral region of the wafer. Thewheel member can further include a fluid delivery system for passingfluid to the inner surface of the jacket, thereby providing fluid whichpercolates or perfuses to the outer surface of the jacket. Such a fluiddelivery system can include a fluid guiding passage having a distal endconnected to a source of fluid and a proximal end connected to anaperture in the rim.

A fuller understanding of the nature and objects of the invention willbe apparent from the following detailed description and the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view, partly cut-away, of a wafer cleaningapparatus according to the invention;

FIG. 2 is a simplified view showing the scrubbing subsystem and brushpivoting mechanism of the wafer cleaning apparatus of FIG. 1;

FIG. 3 is a simplified view showing the wafer rotating subsystem andsupport arm pivoting mechanism from the wafer cleaning apparatus of FIG.1;

FIGS. 4A and 4B are fragmentary views showing how the wafer rotatingsubsystem of FIG. 3 controls the draft of the wafer;

FIG. 5 is a side elevation view of the wafer cleaning apparatus shown inFIG. 1;

FIG. 6 is an exploded view of a rim driving wheel of the wafer rotatingsubsystem of FIG. 3;

FIG. 7 is an exploded view of a brush of the scrubbing subsystem of FIG.1;

FIG. 8 is an isometric view, partly cut-away, showing the fluid deliverymanifold of the brush shown in FIG. 7; and

FIG. 9 is a schematic diagram showing an exemplary flow of threecleaning solution constituents on the brush of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Overview

A wafer cleaning apparatus according to the invention integrates, into asingle housing, several subsystems which cooperate to clean both sidesof a vertically oriented semiconductor wafer.

The housing contains within it a process tank for providing control overthe level of process liquid, if any, into which the wafer is partiallyimmersed during cleaning. A scrubbing subsystem provides brushes formechanically scrubbing the wafer with a pre-selected brush pressure.

The scrubbing subsystem is coupled to a brush arm pivoting mechanism forcontrolling the pressure exerted by the brushes on the wafer. The brushpivoting mechanism includes a feedback loop which enables it toadaptively respond to changes in brush pressure caused by brush wear.

The scrubbing system is also coupled to a fluid delivery subsystem whichprovides for delivery of a plurality of fluids to the cleaning surfacesof the brush. A salient feature of the apparatus is that the fluids aredelivered to the brush surface independently so that mixing of thefluids occurs on the surface of the brush.

To ensure that all parts of the wafer come into contact with thescrubbing subsystem, the wafer is rotated by a wafer rotating mechanism.This wafer rotating mechanism rotates the wafer by engaging itsperiphery and imparting to it a rotational force. A second fluiddelivery subsystem coupled to the wafer rotating mechanism appliescleaning fluid to the periphery of the wafer thereby enabling the waferrotating mechanism to simultaneously rotate wafer and clean thoseportions of the wafer with which it comes into contact.

Finally, the process tank can optionally be coupled to a megasonicsubsystem which can transmit megasonic waves through the process liquidin the process tank. These megasonic waves enhance the cleaning actionof the scrubbing system.

Housing

FIG. 1 shows a plenum tank 84 within which a process tank 82 is mounted.The plenum tank 84 has a lid 88 through which a wafer W can be insertedinto the process tank 82. Within the process tank 82, the wafer W restson supports 86 a-c protruding from the bottom of the tank 82 and bestseen in FIG. 2.

The process. tank 82, shown in cross section in FIG. 2, includes awafer-cleaning section 84 a and a wider brush-cleaning section 84 bformed by a shoulder 83. When process liquid L fills the wafer-cleaningsection 84 a, the wafer W is partially immersed in the process liquid Lwhile the brushes 12 a, 12 b remain above the surface of the processliquid L. When process liquid L fills the brush cleaning section 84 b,the brushes 12 a, 12 b are immersed in the process liquid L.

A drain valve 89 connected to a drain 85 in the shoulder 83 controls thelevel of the process liquid L. When the drain valve 89 is closed, thelevel of the process liquid L rises to fill the brush cleaning section84 b. When the drain valve 89 is open, any process liquid L beyond thatrequired to fill the wafer cleaning section 84 a overflows onto theshoulder 83 and drains away through the drain 85. As a result, when thedrain valve 89 is open, the process liquid L cannot fill the brushcleaning section 84 b.

Scrubbing Subsystem

In a wafer cleaning apparatus 100 embodying the invention, the scrubbingsubsystem 10, as shown in FIG. 1, includes a first brush arm 14 aextending upward from its pivotably mounted end 15 a, which is coupledto a brush pivoting mechanism 20 disposed outside of the process tank82, to its free end 16 a. A second brush arm 14 b opposed to the firstbrush arm 14 a also extends upward from its pivotably mounted end 15 b,which is likewise coupled to the brush pivoting mechanism 20, to itsfree end 16 b. Between the free ends 16 a, 16 b of the first and secondarms extends a first cylindrical brush 12 a having a first end 13 arotatably mounted to the free end 16 a of the first brush arm 14 a and asecond end 13 b rotatably mounted to the free end 16 b of the secondbrush arm 14 b. The major axis of the first cylindrical brush 12 a thusextends between the free ends 16 a, 16 b of the first and second brusharms 14 a, 14 b.

Third and fourth brush arms 14 c, 14 d likewise extend upward from theirpivoted ends 15 c, 15 d, both of which are coupled to the brush pivotingmechanism 20, to their corresponding free ends 16 c, 16 d. The two ends13 c, 13 d of a second cylindrical brush 12 b are likewise rotatablymounted to the free ends 16 c, 16 d such that the major axis of thebrush 12 b extends between the free ends 16 c, 16 d of the third andfourth arms 14 c, 14 d. The third and fourth arms 14 c, 14 d standopposed to the first and second arms 14 a, 14 b such that the major axisof the first cylindrical brush 12 a is parallel with the major axis ofthe second cylindrical brush 12 b.

As shown in the side view of FIG. 2, the first and second cylindricalbrushes 12 a, 12 b define a gap or opening therebetween. The spacebetween these two brushes 12 a, 12 b, and hence the width of this gap,can be adjusted by pivoting the first and second brush arms 14 a, 14 brelative to the third and fourth brush arms 14 c, 14 d. In the waferloading position, shown in FIG. 2, the gap is sufficiently wide topermit a wafer W to be placed into the gap. In the wafer cleaningposition, the brush arms 14 a-14 d are pivoted so as to form a gapsufficiently narrow to enable the brushes 12 a, 12 b to contact thewafer W. In this position, the brushes 12 a, 12 b can exert a pressureagainst the faces of a wafer placed in the gap.

The illustrated brush pivoting mechanism 20 shown in FIG. 2 includes apiston 22 which forms an airtight seal inside a cylinder 21. Theposition of the piston 22 within the cylinder 21 can be adjusted byincreasing the air pressure within the cylinder 21. This air pressure isunder the control of a brush pressure controller 23 which is responsiveto the pressure exerted by at least one of the first and secondcylindrical brushes 12 a, 12 b against the wafer W.

As shown in FIG. 2, the pivotably mounted end 15 a of the first brusharm 14 a is coupled, at a first pivot point 26 a, to a first ear 24 a.In a similar manner, the pivotably mounted end 15 c of the third brusharm 14 c is coupled, at a second pivot point 26 c opposite the firstpivot point 26 a, to a second ear 24 c. The first and second ears 24 a,24 c are pivotably coupled to the piston 22 and cylinder 21respectively. Opposed gear teeth 25, attached to the first and thirdbrush arms 14 a, 14 c respectively, are disposed to engage each othersuch that the first and third brush arms 14 a, 14 c move in acoordinated manner.

It will be appreciated that the second and fourth brush arms 14 b, 14 dcan be coupled in a manner similar to the first and third brush arms 14a, 14 c. If the first and second brush anus 14 a, 14 b are connected bya first connecting rod (not shown) and the third and fourth brush arms14 b, 14 d are connected by a second connecting rod (not shown), it willbe clear to one skilled in the art that a single piston 22 and cylinder21 can control the pressure exerted by the first and second cylindricalbrushes 12 a, 12 b on the two faces of the wafer W. Alternatively, aseparate cylinder and piston can be provided for pivoting the first andthird brush arms 14 a, 14 c independently of the second and fourth brusharms 14 b, 14 d. In such a case, the pressure exerted by the first andsecond brushes 12 a, 12 b can be varied along the chord on the wafer W.

Subsystem for Delivery of Cleaning Solution through the Brush

Because it is advantageous to deliver a cleaning solution to the surfaceof the wafer W during the scrubbing process, a preferred embodiment ofthe wafer cleaning apparatus 100 includes a fluid delivery subsystem 50for delivering a cleaning solution to the brush 12 a. This cleaningsolution is typically a mixture of several fluid constituents which canbe liquids or gases. Examples of such fluid constituents includenitrogen gas (N₂), acids such as hydrofluoric acid (HF), bases such asammonium hydroxide (NH₄OH), and surfactants. In some cases, the cleaningsolution loses its potency soon after its constituents are mixedtogether. For this reason, the preferred fluid delivery subsystem 50mixes the constituents of the cleaning solution as closely as possibleto the point at which the cleaning solution first contacts the wafer W.

FIG. 9 shows a schematic diagram of the fluid delivery subsystem 50 ofthe invention. As shown therein, a first constituent flows through afirst delivery tube 61 a under pressure provided by a pump or otherpressure source (not shown). A first control valve 59 a in the pathdefined by the delivery tube 61 a controls the volume rate of flow ofthe first constituent and thereby makes possible the real-timeadjustment of its concentration in the cleaning solution.

Soon after it enters the interior of the brush 12 a, the firstconstituent flows radially toward the brush surface. Just below thebrush surface, the first constituent flows along an axially directedflow line 62 a parallel to the major axis of the brush 12 a. As it doesso, it diffuses circumferentially from the flow line 62 a, as shown bythe pairs of opposed, circumferentially directed arrows in FIG. 9. Thisresults in an axially directed strip 63 a of the first constituentdiffusing circumferentially on the surface of the brush 12 a. The rateat which this circumferential diffusion occurs depends, in part, on theporosity of the brush surface, on the pressure driving the flow of thefirst constituent, and on the forces generated on the fluid constituentby the rotation of the brush 12 a.

The remaining two constituents of the cleaning solution flow on deliverytubes 61 b, 61 c through control valves 59 b, 59 c in a similar manner.This likewise results in axially directed strips 63 b, 63 c of theremaining two constituents which diffuse circumferentially on thesurface of the brush 12 a.

It is apparent from examination of FIG. 9 that the circumferentiallydiff-using strips 63 a-63 c can overlap with each other as the stripsgrow wider. In those regions in which the diffusing strips overlap,there is mixing between the constituents associated with those strips.Moreover, as the brush rotates, each strip 63 a-63 c contacts the wafersurface at least once per rotation and deposits thereon the componentcarried to that strip by its corresponding delivery tube 61 a-61 c.

In this way, the fluid delivery subsystem 50 maintains the separation ofthe fluid constituents until just before the cleaning solution is to beapplied to the wafer W.

The foregoing method of mixing the constituents of the cleaning solutionon the surface of the brush 12 a can be implemented by the multilayerbrush structure depicted in the partially cutaway view of one end 13 aof a cylindrical brush 12 a in FIG. 8.

As shown in FIG. 8, the brush 12 a is a multilayer structure having, asits outermost layer, a porous jacket 58, preferably made of an open-cellsponge material such as PVA (polyvinyl alcohol) of a type that istypically used for wafer cleaning. The porous jacket 58 envelops aporous perforated sleeve 56 on which a large number of smallperforations are arranged along several axial strips 57 a-57 c. Thenumber of axial strips 57 a-57 c is typically, but need not be, amultiple of the maximum number of constituents in the cleaning solution.

The sleeve 56, in turn, is removably engaged over a hollow brush core 53having several axially directed channels 55 a-55 c cut into its surface.Each such channel 55 a has at least one channel aperture 54 a on itsfloor through which a constituent fluid can enter the channel. In theillustrated embodiment, each channel 55 a is aligned with acorresponding axial strip 57 a on the perforated sleeve 56. As a result,fluid entering the channel 55 a through the channel aperture 54 a canexit the channel 55 a through the perforations making up itscorresponding axial strip 57 a.

A manifold housing 51 carrying three stationary manifolds 61 a-61 cpenetrates the space within the hollow brush core 53. Each manifold 61 ais a hollow tube directed axially through the manifold housing 51. At apoint proximate to a corresponding channel aperture 54 a, the manifold61 a bends radially before terminating in a manifold exit aperture 52 a.At least once per rotation of the brush 12 a, the manifold exit aperture52 a is in fluid communication with the channel aperture 54 a. In thepreferred embodiment, there are three channel apertures 54 a spacedcircumferentially 120 degrees apart on the core 53. Thus, in thepreferred embodiment, the manifold exit aperture 52 a is incommunication with a channel aperture 54 a three times per rotation ofthe brush 12 a. It will be appreciated that the remaining two manifolds61 b, 61 c and their corresponding channel apertures 52 b, 52 c havesimilar structures and functions.

The exploded view of FIG. 7 shows the remainder of the channel 55 aextending along the major axis of the cylindrical brush core 53. Thechannel aperture 54 a through which fluid can enter the channel 55 a isshown at one end of the channel 55 a. Additional channels 55 b-55 c areshown extending along the outer surface of the brush core 53. Thechannels 55 a-55 c are aligned with corresponding rows of perforations57 a-57 c in the sleeve.

While the illustrated embodiment shows a fluid delivery system 50 fordelivery of three constituents, it is readily apparent that only slightmodifications are required to deliver a smaller or larger number ofconstituents. Additionally, variations in the placement of theillustrated apertures and channel, in the choice of which apertures andchannels will deliver which constituent, and in the shapes of theapertures and channels are considered minor modifications which fallwell within the scope of the appended claims.

Referring to FIGS. 8 and 9, whenever a channel aperture 54 a is alignedwith, and therefore in fluid communication with, a manifold exitaperture 52 a, a constituent fluid flowing in the manifold 61 a can exitthe manifold 61 a through its exit aperture 52 a and enter the channel55 a by passing through the channel aperture 54 a. Similarly, twoadditional constituent fluids can flow, independently of any otherconstituent fluids, through the remaining manifolds 61 b-61 c and outthrough corresponding exit apertures 52 b, 52 c to their correspondingchannels 55 b-55 c on the surface of the brush core 53.

The flow of a constituent fluid through the manifold can be pulsatile,such that a pulse of constituent is released through the manifold exitaperture 52 a only when the manifold exit aperture 52 a is aligned withthe channel aperture 54 a. Alternatively, the flow of constituent can beconstant, in which case a tight fit between the manifold exit aperture52 a and the inner wall of the core 53 can be relied upon to preventconstituent fluid from flowing when the channel aperture 55 a is notaligned with the manifold exit aperture 52 a.

It will be appreciated that the three constituent fluids filling thechannels 55 a-55 c remain distinct and that, in this way, theillustrated fluid delivery system 50 can deliver up to threeconstituents of a cleaning solution close to the surface of the brush 12a without allowing them to mix prematurely. It will also be appreciatedthat the relative proportions of constituents comprising the cleaningsolution can readily be adjusted by adjusting the valves 65 a-65 c. Thisfeature permits the pressures driving the flow of constituent fluids ineach of the three manifolds 61 a-61 c to be varied independently of eachother.

The constituent fluids filling the three channels 55 a-55 c are forcedthrough their corresponding rows of perforations 57 a-57 c in the sleeve56. After passing through the perforations 57 a-57 c, the constituentfluids perfuse radially through the porous jacket 58 surrounding thesleeve 56. While in the porous jacket 58, the constituents mix and formthe cleaning solution which is ultimately transferred from the outersurface of the porous jacket 58 to the surface of the wafer W.

In this manner, the cleaning solution is formed from its constituents ata point proximate to the wafer surface. This feature of the inventionpermits application of unstable cleaning solutions that lose theireffectiveness shortly after they are mixed. Additionally, because thecombined volume of constituents in the manifolds 52 a-52 c and in thechannels 55 a-55 c is small, the composition of the cleaning solutioncan be rapidly varied.

Wafer Rotating Subsystem

With reference to FIGS. 1 and 3, a wafer cleaning device embodying theinvention includes a wafer rotating subsystem 30 which rotates the waferW as the scrubbing subsystem 10 brushes the wafer. The illustrated waferrotating subsystem 30 includes a first support arm 34 a extending fromits pivotably mounted end 36 a, which is coupled to a support armpivoting mechanism 40 mounted outside the process tank 82, to its freeend 35 a. A first rim driving wheel 32 a is rotatably mounted to thefree end 35 a of the first support arm 34 a such that its axis ofrotation 31 a is parallel to the axis about which the first support arm34 a pivots.

A second support arm 34 b likewise extends from its pivotably mountedend 36 b which, like the pivotably mounted end 36 a of the firstsupporting arm 34 a, is coupled to the support arm pivoting mechanism40. A second rim driving wheel 32 b is mounted to the free end 35 b ofthe second support arm 34 b such that its axis of rotation 31 b isparallel to the axis of rotation 31 a of the first rim driving wheel 32a. Each support arm 32 a, 32 b encloses a belt 45 a, 45 b, shown in FIG.5, which is trained around a driving pulley 46 a, 46 b attached to amotor (not shown) and a driven pulley 47 a, 47 b attached to the rimdriving wheel 32 a, 32 b

The illustrated support arm pivoting mechanism 40, shown in FIG. 3,includes a motor 43 which turns a driving pulley 42 on which is traineda belt 44 that engages a driven pulley 41. The driven pulley 41 iscoupled to rotate two opposed worm gears 39 a, 39 b. These gears arecoupled to matching gears on opposed ears 38 a, 38 b. The ears 38 a, 38b are coupled to pivot the supporting arms 34 a, 34 b about two pivotpoints 36 a, 36 b, respectively. The axes about which the support arms34 a, 34 b pivot are therefore orthogonal to the axes about which thebrush arms 14 a-14 d pivot.

As shown in FIG. 3, the support arms 34 a, 34 b can be pivoted between acleaning position 37 a, shown in solid lines, and a loading position 37b, shown with broken lines. In the cleaning position 37 a, the first andsecond rim driving wheels 32 a, 32 b are disposed to frictionally engagethe wafer W at first and second engagement sites 33 a, 33 b along theperiphery of the wafer W. In this cleaning position 37 a, rotation ofthe rim driving wheels 32 a, 32 b imparts a rotational motion to thewafer W about an axis orthogonal to the major axes of the first andsecond cylindrical brushes 12 a, 12 b. By varying the angle θ at whichthe support arms 34 a, 34 b are pivoted, as shown in FIGS. 4A and 4B,the length l of the chord separating the two engagement sites 33 a, 33 bcan be adjusted. This, in turn, controls the draft d of the wafer Wrelative to the brushes 12 a, 12 b.

When the angle θ is large, as shown in FIG. 4A, the wafer W rises agreater distance before its periphery contacts the rim driving wheels 32a, 32 b. This decreases the draft d₁. Conversely, when the angle θ issmall, as shown in FIG. 4B, the periphery of the wafer W contacts therim driving wheels 32 a, 32 b sooner. This increases the draft d₂.

In the loading position, the rim driving wheels 32 a, 32 b are no longerover the gap. Consequently, in this position, a wafer W can be eitherplaced into or removed from the gap between the first and secondcylindrical brushes 12 a, 12 b without interference by the rim drivingwheels 32 a, 32 b.

Subsystem for Delivery of Cleaning Solution to Wafer Periphery

In one embodiment, the rim driving wheels 32 a, 32 b include a fluiddelivery subsystems 60 a, 60 b for delivering cleaning solution throughthem and onto the point at which they engage the wafer W. FIG. 6 showsan exploded view of the first rim driving wheel 32 a and its fluiddelivery subsystem 60 a. The second rim driving wheel 32 b preferablyincludes a similar fluid delivery system and hence has the sameconstruction.

As shown in FIG. 6, the fluid delivery subsystem 60 a for the rimdriving wheel 32 a includes a fluid delivery tube 69 having an aperture65. The fluid delivery tube 69 slides through central holes (not shown)in a first compression disk 67 a, in a second compression disk 67 b, andin a porous jacket 66 sandwiched between the first and secondcompression disks 67 a, 67 b. The porous jacket 66 can be a porous oropen cell sponge material, such as a PVA sponge, of the type used inconnection with the cylindrical brushes 12 a, 12 b. An adjustable diskcompressor 68 at the end of the fluid delivery tube 69 exerts a forcealong the axis of the fluid delivery tube 69 and toward the aperture 65,thereby compressing the porous jacket 66 between the first and secondcompression disks 67 a, 67 b.

In an embodiment having a fluid delivery subsystem 60 a as describedabove, cleaning solution flowing through the fluid delivery tube 69exits the tube 69 through the aperture 65. Fluid exiting the aperture 65then contacts the inner surface of the porous jacket 66 and perfuses tothe outer surface of the jacket 66 and onto the periphery of the waferW. As a result, instead of leaving contaminants on the periphery of thewafer W, as do conventional rim driving wheels, the rim driving wheels32 a, 32 b of this embodiment clean the periphery of the wafer W as theyrotate the wafer W.

Megasonic Transducer Subsystem

It has been found that the cleaning efficiency of the scrubbing processset forth above can be further enhanced by bombarding the surface of thewafer with megasonic waves during the scrubbing process. Accordingly, apreferred embodiment of the wafer cleaning apparatus 100, as shown inFIG. 5, includes a megasonic transducer subsystem 70 disposed such thatwhen the process tank 82 contains process liquid L, the megasonictransducer subsystem 70 is below the surface of that process liquid L.As shown in FIG. 5, the megasonic transducer subsystem 70 can consist ofone or more megasonic transducers 71 a-71 d disposed flush with thebottom of the process tank 82 to avoid interference with any movingparts. However, it is understood that the megasonic transducer subsystem70 can include megasonic transducers 71 a-71 d mounted on one or moredifferent walls of the process tank 82 or in any other location in whichit does not interfere with the mechanical operation of the wafercleaning apparatus 100.

The megasonic transducers 71 a-71 d that constitute the megasonictransducer subsystem 70 can be operated continuously or intermittentlyduring the cleaning cycle. The individual transducers 71 a-71 d can beoperated simultaneously, either in phase or with a progressive phasedifference between them. Alternatively, the individual transducers 71a-71 d can be switched on and off in a pre-defined pattern.

The effectiveness of megasonic cleaning can be further enhanced byelevating the temperature of the process liquid L through which themegasonic waves propagate. Accordingly, a preferred embodiment includesa temperature controller 87 for maintaining the temperature of theprocess liquid L at a pre-selected operating temperature. A preferredoperating temperature for megasonic cleaning is in the range from 40° C.to 60° C.

The frequency at which the transducers 71 a-71 d are operated isselected to maximize the likelihood that contaminants will be dislodgedfrom the surface of the wafer W. These frequencies are typically in therange between 600 kHz and 1000 kHz. The transducers can be operated at asingle frequency throughout the cycle, or at selected frequencies fordifferent intervals during the cleaning cycle. Additionally, one or moretransducers can sweep across the megasonic frequency range eitherlinearly in time or according to a pre-selected time varying function.

Operation of the Preferred Embodiment

The wafer cleaning apparatus 100 offers three modes of operation: awafer cleaning mode in which a wafer is scrubbed; an enhanced wafercleaning mode in which the wafer is both scrubbed and subjected tomegasonic energy; and a brush cleaning mode in which the wafer is absentand the brushes are exposed to megasonic energy.

In both the wafer cleaning mode and in the enhanced wafer cleaning mode,the brush pivoting mechanism 20 pivots the first and second brush arms14 a, 14 b apart from the third and fourth brush arms 14 c, 14 d so asto increase the gap between the first and second brushes 12 a, 12 b.Simultaneously, the support arm pivoting mechanism 40 pivots the supportarms 34 a, 34 b apart and into the loading position 37 b shown in brokenlines in FIG. 3. A wafer W is then inserted, typically by a robot arm(not shown), into the gap thus formed between the first and secondbrushes 12 a, 12 b. After insertion into the gap, the wafer W rests onsupports 86 a-86 c, best seen in FIG. 3, protruding from the bottom ofthe process tank 82.

Once the wafer W is loaded into the process tank 82, the brush pivotingmechanism 20 then pivots the brush arms 14 a-14 d such that the firstand second brushes 12 a, 12 b contact and exert a selected pressure onthe front and back surfaces of the wafer W. Simultaneously, the supportarm pivoting mechanism 40 pivots the support arms 34 a, 34 b into thecleaning position 37 a, shown in solid lines in FIG. 3.

During the cleaning cycle, the first and second brushes 12 a, 12 brotate about their respective major axes in opposite directions. Thisrotation imparts to the wafer a generally upward force that lifts thewafer W from the supports 86 a-86 c and up against the rim drivingwheels 32 a, 32 b. At the same time, the rim driving wheels 32 a, 32 b,which now engage the periphery of the wafer, limit the upward motion ofthe wafer W caused by the rotation of the brushes 12 a, 12 b. Theinteraction between the upward force exerted by the brushes 12 a, 12 bon the wafer W and the downward force exerted by the rim driving wheels32 a, 32 b controls the vertical position of the wafer W relative to thebrushes 12 a, 12 b. Additionally, the rim driving wheels 32 a, 32 b aredriven to rotate the wafer about an axis passing through its center andorthogonal to its faces. If desired, the wafer cleaning section 84 a ofthe process tank 82 can be filled with a process liquid L to a levelclose to but not in contact with the surfaces of the brushes 12 a, 12 bso that as the wafer W rotates, each section of the wafer W isalternately immersed in and withdrawn from the process liquid L.

In the enhanced wafer cleaning mode, the megasonic transducer subsystem70 generates megasonic waves in the process liquid L concurrently withthe operation of the brushes 12 a, 12 b and the rim driving wheels 32 a,32 b. Thus, in this mode, the coordinated operation of these threecomponents results in the wafer cleaning apparatus 100 scrubbing eachportion of the wafer W, dipping the recently scrubbed portion into theprocess liquid L for exposure to megasonic energy to loosen anyparticles remaining on the wafer W, and raising that portion back out ofthe process liquid L in order to scrub off the particles loosened byexposure to the megasonic waves in the process liquid L. This process ofrepeatedly scrubbing each portion of the wafer W and dipping it into theprocess liquid L for exposure to megasonic waves enhances the cleaningefficiency of the apparatus by exploiting the synergistic effect of nearsimultaneous application of brush scrubbing and megasonic bombardment.Because the mechanical components of the brushes 12 a, 12 b are allisolated from the process liquid L, contamination of the process liquidL is minimized.

In the brush cleaning mode of operation, the wafer W is removed from theprocess tank 82, the drain 85 in the shoulder 83 of the process tank 82closes, and the brush cleaning section 84 b is filled with processliquid L to a level high enough so that a portion of each brush 12 a, 12b is immersed in process liquid L. The brushes 12 a, 12 b are thenrotated about their respective axes. Concurrent with the operation ofthe brushes, the megasonic transducer subsystem 70 generates megasonicwaves in the process liquid L. As a result, during each rotation of thebrushes 12 a, 12 b at least a portion of each brush 12 a, 12 b dipsbelow the level of the process liquid L. As a result, the brushes 12 a,12 b are repeatedly exposed to megasonic bombardment. This has theeffect of cleaning the brushes 12 a, 12 b, thereby prolonging theiruseful life and further enhancing their cleaning effectiveness duringoperation of the wafer cleaning apparatus 100 in either the wafercleaning mode or in the enhanced wafer cleaning mode.

The extent to which the wafer W is above or below the brushes 12 a, 12 b, hereafter referred to as the draft, can be controlled by varying thepositions of the engagement sites 33 a, 33 b. As shown in FIG. 4A, whenthe engagement sites 33 a, 33 b are far apart, the rim driving wheels 32a, 32 b do not exert a downward force on the wafer W until the brushes12 a, 12 b have already lifted it relatively high. As a result, in thisposition, the wafer's draft d₁ is small. Conversely, when the engagementsites 33 a, 33 b are closer together, as shown in FIG. 4B, the rimdriving wheels 32 a, 32 b exert a downward force on the wafer W beforethe brushes 12 a, 12 b have lifted it very high. As a result, in thisposition, the wafer's draft d₂ is relatively large.

During the cleaning cycle, the brush pressure controller 23 periodicallygenerates a correction signal by measuring the pressure exerted by thebrushes 12 a, 12 b on the wafer W and comparing it with a pre-selectedpressure. Variations in brush pressure can arise from non-uniformitiesin the radii of the cylindrical brushes 12 a, 12 b. Additionally, as thecylindrical porous jacket 58 wears, it becomes thinner. Consequently,the pressure exerted by the brushes 12 a, 12 b when the brush arms 14a-14 d are at a particular location relative to the wafer will tend todecrease as the porous jacket 58 wears.

The correction signal thus generated is fed back to the brush pivotingmechanism 20. If this measured pressure exerted by the brushes 12 a, 12b is too low, the brush pressure controller 23 signals the brushpivoting mechanism 20 to pivot the free ends of the brush arms 14 a-14 dinward toward the wafer W, thereby reducing the gap between the brushes12 a, 12 b and increasing the pressure exerted on the wafer W.Conversely, if the measured pressure is too high, the brush pressurecontroller 23 signals the brush pivoting mechanism 20 to pivot the freeends of the brush arms 14 a-14 d outward and away the wafer W, therebyincreasing the gap between the brushes 12 a, 12 b and reducing thepressure exerted on the wafer W.

After a selected cleaning interval, both the brush arms 14 a-14 d andsupporting arms 34 a, 34 b pivot back into their respective waferloading positions. The scrubbed wafer W can then be lifted out of theprocess tank 82, typically by a robot arm (not shown), for furtherprocessing by, for example, a surface tension gradient drying stage.

If necessary, the cylindrical porous jacket 58 can be replaced bydisengaging the sleeve 56 from the brush core 53 and replacing it with asleeve on which a porous jacket has been pre-mounted. This eliminatesthe difficulty inherent in attempting to slide the porous jacket 58,which is typically a cylinder made of a soft, sponge-like material, ontoa core 53 having an outer diameter larger than the inner diameter of theporous jacket 58.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

It will thus be seen that the invention efficiently overcomes thedisadvantages set forth above. Since certain changes may be made in theabove constructions without departing from the scope of the invention,it is intended that all matter contained in the above description orshown in the accompanying drawings be interpreted as illustrative andnot in a limiting sense.

It is also to be understood that the following claims are intended tocover all generic and specific features of the invention describedherein, and all statements of the scope of the invention which as amatter of language might be said to fall therebetween. Having describedthe invention, what is claimed as new and secured by Letters Patentis:
 1. In a wafer cleaning apparatus, a brush assembly comprising: abrush core having an outer surface and a channel axially extending alongthe outer surface thereof and distal from a center axis thereof, saidchannel having a bottom surface inset from said outer surface; a firstcylindrical sleeve coaxial with said brush core, said sleeve beingremovably engaged with said core; and a second cylindrical sleeve havinga brush surface coaxial with said first cylindrical sleeve, saidcylindrical sleeve being engaged with said first cylindrical sleeve;wherein the brush core and the first and second cylindrical sleeves aresized and dimensioned for mounting in the wafer cleaning apparatus andwherein the channel is configured for allowing passage of a fluid alonga length of said brush core and to the first and second cylindricalsleeves.
 2. The brush assembly of claim 1 wherein said first cylindricalsleeve has a perforated radial wall.
 3. The brush assembly of claim 1wherein said first cylindrical sleeve includes a first end having a tabprotruding axially therefrom, said tab dimensioned to engage acorresponding notch in said brush core.
 4. The brush assembly of claim 1wherein said first cylindrical sleeve includes a first end having anotch extending axially therein, said notch dimensioned to engage acorresponding tab in said brush core.
 5. The brush assembly of claim 1wherein said brush surface is a porous jacket.
 6. The brush assembly ofclaim 5 wherein said porous jacket is made of an open cell spongematerial.
 7. The brush assembly of claim 6 wherein said open cell brushmaterial is PVA.
 8. In an apparatus for scrubbing a planar substrate, areplaceable brush assembly comprising: an elongated brush core having anouter surface and a channel extending along the outer surface thereofand distal from a center axis thereof, said channel having a bottomsurface inset from said outer surface; a tubular sleeve having an innersurface and an outer surface, said sleeve defining a lumen fortelescopically receiving said elongated brush core such that said innersurface of said sleeve faces said brush core outer surface; means forremovably and replaceably engaging said tubular sleeve for rotation withsaid elongated brush core; and a tubular scrubbing medium having aninner surface engaged with said outer surface of said tubular sleeve;wherein the brush core and the first and second cylindrical sleeves aresized and dimensioned for mounting in the wafer cleaning apparatus andwherein the channel is configured for allowing passage of a fluid alonga length of said brush core to the tubular sleeve and the tubularscrubbing medium.
 9. The replaceable brush assembly of claim 8 whereinsaid engaging means comprises a tab protruding from an end of saidtubular sleeve, said tab being dimensioned to engage a correspondingnotch in said elongated brush core.
 10. The replaceable brush assemblyof claim 8 wherein said engaging means comprises a notch formed in oneend of said tubular sleeve, said notch being dimensioned to engage acorresponding tab in said elongated brush core.
 11. The replaceablebrush assembly of claim 8 wherein said tubular sleeve is perforated by amultiplicity of passageways providing fluid communications between saidinner surface and said outer surface.
 12. The replaceable brush assemblyof claim 8 wherein said tubular scrubbing medium is a porous jacket.